Somebody guide me to implement tag subscription plugin.

The way I would approach this is:

• Create several AJAX calls using the nopriv hook bound to the subscribe/follow button clicks.
• Within the PHP callback functions, I would insert value of the tag or the followed user into the logged-in user’s meta using add_user_meta, get_user_meta, delete_user_meta.
• Lastly, I would create a conditional function to use in your templates that pulls down the user meta, checks the current tags or users and then return a bool value so you can render the proper state of your buttons.

The AJAX PHP callback would use something like the following:

global $current_user;
get_currentuserinfo();
$tags = get_user_meta($current_user->ID, 'tag_subscription');
if(in_array($tag_slug, $tags))
    //do something
else
    add_user_meta($current_user->ID, 'tag_subscription', $tag_slug, false);

To remove an element from the user meta array, you would do the following:

delete_user_meta($current_user->ID, 'tag_subscription', $tag_slug);

The conditional template tag would look like this in your template:

if(is_user_subscribed()){
     //display this state of the button
}
else{
     //display another state of the button
}

Hope this helps you out!